CODEWITHSUNDEEP
lisp
user1046935
user1046935

Reputation:

A complex Lisp issue

Define a function contains that takes two parameters, a symbol A and a list of symbols L, and returns t only if L contains A as one of its elements.

That is the original question. I am very new to LISP and would like some code along with an explanation of the syntax. Coming from a java and C++ background LISP is very different in nature. Please help.

So far my code successfully takes two inputs as an parameter.

Pseudocode: I compare the value in A with the first Value of the List then pass the list and variable A back into the function with a recursive call until the the List is empty. If the List is empty return null.

The issue with this is if List contains the a nested List how do i recognize the nested loop.

Such as: (contains #\b '(a (((b))) c)) -> must print true.

So Far What I Have Code:

(defun contains (a list)
    (if (eq a(car list))
        (princ "t")
        (contains a (cdr list))))

(contains  #\B '(B a c d e f g))

I need a way for detecting at the end of the list. And a way to search inside nested lists.

Upvotes: 0

Views: 98

Answers (1)

sheikh_anton
sheikh_anton

Reputation: 3452

First of all let's fix some your mistakes: #\b isn't a SYMBOL, it's a CHARACTER, as I understand we looking for symbol in a list of symbols, so we don't need to do some character -> symbol conversion. Second: don't (princ t) just return t.

My short variant:

(defun contains (sym nested-list)
  "Find if NESTED-LIST contains a SYM"
  ;; Check if our list is CONS-cell?.
  (if (consp nested-list)
    ;; Recursion on cars and cdrs
    (or (contains sym (car nested-list))
        (contains sym (cdr nested-list)))
    ;; Elseway just check for equality
    (eq sym nested-list)))

CL-USER> (contains 'a '(b c (x b y (f a)) d))
T
CL-USER> (contains 'w '(b c (x b y (f a)) d))
NIL

On your test-case too:

CL-USER> (contains 'b '(a (((b))) c))
T

Upvotes: 2

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